In typewriter printing applications there are several instances in which the active marking region of the ribbon is to be raised and lowered with respect to the platen: (1) when it is necessary for the operator to be able to see the typed print line which is obscured from view during the printing action; (2) to shift a multicolored ribbon to permit print color selection; and (3) to obtain optimum utilization of ribbon material by impacting the ribbon at vertically distinct positions thereon prior to advancing the ribbon laterally. To this end interactive keyboard printer devices have been provided with various mechanisms for raising and lowering the ribbon.
Conventionally the ribbon cartridge is mounted upon a platform in the machine and suitable drives and linkages are included to oscillate the entire platform and cartridge in timed relation to character impacts. This solution requires the rapid movement of cumulatively heavy elements, creating momentum and inertia problems. As a result, expensive motors and controls must be provided in order to achieve accurate movements.
One successful solution to minimizing the momentum and inertia problems associated with the vertical movement of the active marking region of the ribbon has been the design of ribbon cartridges with pivotable ribbon support arms. In embodiments of this type, only the arms, which carry the active region of the ribbon therebetween, need be vertically oscillated or vibrated. The much lower mass can be more rapidly moved with significantly less expensive mechanisms and the generation of objectionable noise.
Shifting of the cartridge arms creates a major problem because the active marking portion of the ribbon is moved relative to its supply and take-up portions about a pivot axis which is parallel to the active marking portion. This results in the generation of uneven stresses in the ribbon due to unequal lengths of travel of its edges. Usually there will be one taut edge and one slack edge, a condition which results in the folding over of the slack edge, a decrease in the usable ribbon width causing incomplete typed characters and eventual fouling of the ribbon transport.
U.S. Pat. No. 4,397,575 (Aldrich) and U.S. Pat. No. 4,402,621 (Abell, Jr. et al) disclose ribbon cartridges with vibrating ribbon guide arms. In each of these, edge tensions develop, caused by the different lengths of ribbon paths of the top and bottom edges as the arms are shifted. Fold over can be a consequence of these shifting configurations.
An attempt to solve the problem is suggested in U.S. Pat. No. 4,423,973 wherein a ribbon raising and lowering device is disclosed in which the pivot arm has a long radius from the pivot point to the ends of the movable guide arms defining the active marking region. Ribbon tension is reduced, by diminishing the variation in ribbon length between the top and bottom edges, but still is present.
Usually motion imparting means need only push the arms upwardly since downward biasing means provided within the cartridge (as in U.S. Pat. No. 4,402,621) acts to normally hold the arms in one direction. Thus, in its free state, out of the machine, the bottom edge of the ribbon normally is slack. In this condition, any advancement of the ribbon will cause the ribbon to slide toward the slack edge and to fold. An adverse condition occurs upon removing the cartridge from its shipping container. The ribbon in the active marking region will be a little loose, and single strike ribbon will probably will have some ink voids where handling has occurred. Any ink voids will result in incomplete printing. Immediately prior to introduction into the machine, it is usually necessary for the operator to manually advance the ribbon. Operator instructions, packaged with the cartridge, caution the operator to lie the cartridge on a flat surface, so as to straighten the pivotable arms, prior to manually advancing the ribbon, so that a taut, completely inked, portion extends across the active marking region. This will prevent fold-over because, in this condition, both edges will follow a path of the same length. However, since operators do not always follow instructions, fold-over often occurs during the manual advancing and subsequent machine advancement continues to fold the ribbon and it eventually fouls itself.